Electric immersion heaters are manufactured in many forms for use in heating various liquids. All immersion heaters have a tubular metal sheath in which a heating resistor coil is disposed, the coil being electrically insulated from the sheath by compacted refractory material which also serves to conduct the heat from the coil to the sheath.
The opposite ends of the resistor coil are mechanially and electrically secured to terminal pins which extend through insulator bushings in the opposite ends of the sheath for connection to a source of electrical energy. The opposite ends of the sheath are mechanically connected to a support which may take the form of a flange mounting plate, a screw plug, a junction box and the like.
A common type of immersion heater is used for heating water in a hot water tank in houses for the supply of hot water. This type of heater may take the form shown in U.S. Pat. Nos. 3,217,138; 3,585,359; and 3,778,592, all assigned to the assignee of the present invention, and comprises a flat metal flange having a pair of holes through which the opposite ends of a hairpin sheath are disposed. The opposite sheath ends are firmly connected to the plate by welding, brazing or staking.
Other forms of immersion heaters are disclosed in the Industrial Stock Catalog entitled "Chromalox" and distributed by the Edwin L. Wiegand Division of Emerson Electric Co. Included in these other forms are screw-plug type elements, having either a hair-pin sheath or a cartridge-type sheath; heater for coffee urns and the like; over-the-side heaters; circulation heaters; and the like.
It has heretofore been customary to form the sheath of the immersion heaters of relatively expensive copper, stainless steel, Incoloy or other suitable metals, to resist the corrosive action of the liquid in which the sheath of the immersion heater is immersed.
In the case of the usual heater for a hot water tank, the sheath is commonly made of copper or copper alloy. To prolong the life of the magnesium anode rod commonly installed in hot water tanks, the copper sheath is tin-plated, and this represented a further expense.
It has heretofore been proposed to form the sheath of an immersion heater of aluminum because of the relatively lower cost of this material and the ease with which it may be worked. However, aluminum has a tendency to hydrate when immersed in water, and to build what is commonly termed as "sores," namely, white spots which build up and spall off until a hole is eventually formed in the sheath and this, of course, results in failure of the heating element.
Tests have also been conducted wherein the aluminum sheath was tin-plated, but although this was an improvement, these methods of putting tin on aluminum used an intermediate layer of zinc which was attacked and dissolved by galvanic action, which in turn caused the tin coating to blister and fall off. I have discovered that if an adherent base is applied to the aluminum sheath, the tin-plate may thereafter be applied to the sheath to completely cover all exposed surfaces thereof. The adherent base found suitable is a strike in the form of an extremely thin coating of bronze or copper. Prior to this invention, it was not possible to produce a satisfactory immersion heater for heating liquids, such as water, utilizing an aluminum sheath, but this has now been overcome through use of this invention.